1. Field of the Invention
The present invention relates to optical transmitting technique, and in particular, to an optical transmitting apparatus and optical control method of amplifying signal light.
2. Description of the Related Art
In an optical communication system, a wavelength division multiplexing (WDM) communication for increasing communication capacity has prevailed. Therefore, development of a WDM transmitting apparatus has been very brisk. The WDM transmitting apparatus has used an optical-fiber amplifier to extend communication distance by amplifying directly a WDM optical signal. Most optical-fiber amplifiers as a whole have been used to maintain an output level constant. However, due to an optical-fiber amplifying unit therein keeping gain constant, the flatness of an outputted signal light becomes difficult to receive influence. For that reason, the output of the entire optical fiber amplifier needs to be maintained constant, irrespective of change in input level with the gain of the amplifier kept constant.
As the related art, a configuration in which an optical amplifier is provided with a variable optical attenuator is known, as shown in “Japanese Patent Laid-Open No. 2001-144352,” pp. 3 to 5, FIG. 2. In this invention, a variable optical attenuator is inserted between optical amplifying units. By adjusting the attenuation in the variable optical attenuator, the output of the optical amplifier is kept constant even if an input level changes.
FIG. 13 shows the related art. First, signal light inputted into an optical amplifier 200 is amplified by a first amplifying unit 21, attenuated by a variable optical attenuator 24 to a specified level, and outputted. Next, the signal light is again amplified by a second amplifying unit 22, passed through a gain equalizer 25 and a dispersion compensator 26, further amplified by a third amplifying unit 23, and outputted from the optical amplifier 200.
The related art, however, has the following problem.
When the input level of the signal light changes greatly, attenuation in the variable optical attenuator 24 needs increasing. This causes loss to concentrate in one point on a signal path. Concentration of loss in one point lowers the level of the signal at the point. Amplifying the lowered signal light by the optical amplifying unit of the next stage increases the quantity of amplified spontaneous emission (ASE) light. Consequently, an increase of the ASE light causes the noise figure (NF) to deteriorate.
This deterioration is described in detail with reference to FIGS. 14 to 17. FIG. 14 shows an example of a spectrum of signal light inputted into an optical amplifier, of which input level is in steady state. FIG. 15 shows gain and noise figure (NF) value with respect to wavelengths of the amplified signal lights. Next, FIG. 16 shows the cases where the signal lights with two different input levels are input to the optical amplifier by change in input level. When the signal light A is inputted, an attenuation of 5 dB needs to be given by the variable optical attenuator because the signal power is higher by 5 dB than the input level in steady state shown in FIG. 14. Similarly, when the signal light B is inputted, an attenuation of 10 dB needs to be given by the variable optical attenuator because the signal power is higher by 10 dB than the input level in steady state shown in FIG. 14. FIG. 17 shows NF values versus wavelength of the amplified signal lights A and B. It shows that when the signal light B, in particular, is inputted, a loss of 10 dB concentrates in the variable optical attenuator, leading to deterioration in noise figure. Thus, the related art has a problem in that a greater change in the input level causes loss to concentrate, deteriorating noise figure.
Another related art has configuration in which a variable optical attenuator is disposed on a plurality of points on the signal path to prevent loss from concentrating, as shown in “Japanese Patent Laid-Open No. 2003-258346,” pp. 3 to 9, FIG. 1. This disposition disperses loss.
FIG. 18 shows another related art. An optical amplifier 300 is provided with a plurality of gain controlling units 310 and 320. The gain controlling unit 310 has a variable optical attenuator 312 between two optical amplifying units 311 and 313. The gain controlling unit 320 has a variable optical attenuator 322 between two optical amplifying units 321 and 323. The optical amplifier 300 is equipped with an attenuation controlling unit 340 for controlling the variable optical attenuators and a targeted-gain setting unit 350 for controlling optical amplifying units. The attenuation controlling unit 340 controls attenuation of the variable optical attenuators 312 and 322 so that respective output levels of the gain controlling unit 310 at the front stage and the gain controlling unit 320 at the rear stage can be kept constant. The targeted-gain setting unit 350 distributes gain to a plurality of the optical amplifying units 313 and 323 so that noise figure can be optimized when variation arises in a level inputted into this apparatus and in loss quantity between the stages. Therefore, the targeted-gain setting unit 350 controls a second and a fourth optical amplifying units 313 and 323 by setting the optical amplifying units for targeted gains respectively so that the total gain of the four optical amplifiers 311, 313, 321, and 323 is kept constant.
The above related art, however, has the following problem.
In the art, attenuation is controlled separately for each of a plurality of the variable optical attenuators and amplification is controlled separately for each of a plurality of the optical amplifying units as well. That makes control very complicated. In addition, the amplifier needs a plurality of the variable optical attenuators, increasing the number of components, which requires much space for its installation.